LLDP changes in systemd 258

[peckato1]

Systemd fixed sending machine-id as chassis-id over lldp. The patches will be available in systemd 258.
For sd-258 we should fetch the local chassis id DBus from the Describe method of the manager.
For older systemds, we should still fallback to reading the /etc/machine-id

[jktjkt]

Yup, the implementation should simply call out to that method, and if no data are returned or when the call fails, read /etc/machine-id.